Composite structures are being used with increasing frequency as structural elements to provide mechanical support or linkage in environments where metal members were previously used. A typical composite structure is formed from high-strength fibers that are woven together and embedded in matrix material to form a structural member. Some composite structures are, for example, formed of carbon fibers that are imbedded in a epoxy resin matrix. An advantage of composite structures over identically shaped metal pieces is that the composites have the same or greater mechanical strength as the metal pieces yet weigh substantially less. Still another advantage of composite structures is that they can cost much less to manufacture than the available high-strength, low-weight metal alloys fabricated for the same purpose. Thus, the use of composite structures for support members, linkages and the like is becoming very popular in the aerospace industry and other industries where both weight and cost economization are important design considerations.
One factor that has limited the use of composite structures is that it has proved difficult to attach composites to other structural elements. Composite structures, like many other structural elements, are secured to other structural elements by metal pieces known as fittings. The fittings, in combination with other, complementary, fittings or fasteners, secure the individual structural elements together. There are a number of reasons why attaching composite structures has been a formidable task. Many composite structures cannot simply be drilled so that a fitting or fastener can be installed in the drilled hole. This is because loads, and more specifically, bearing or compression loads, are transferred by the fitting directly to the matrix material adjacent the hole. The matrix material often has a low bearing strength and will fatigue under the force of this load. Moreover, the drilling cuts the fibers in the vicinity of the hole, which causes the composite to lose strength.
Other problems arise when providing composite structures with fittings because many composites, especially those formed of fiber, though quite strong in tension are relatively weak in compression. In other words, many composites, while quite strong when subjected to pulling or tension load, have a tendency to break down when subjected to a bearing load. Thus, composites provided with bushings or other fittings tend to be quite strong and wear-resistant as long as the fittings and adjacent structures are subjected to tension loads. However, when the same composite and fitting assemblies are subjected to a compression load, the strength of the assemblies are limited by the bearing strength of the composite material. If these composite structures are subjected to sufficient bearing loads, they may eventually fracture and be useless for the purpose for which they were intended.
A number of techniques have been tried to provide composite structure-fitting assemblies that are resistant to bearing loads. One technique has been to provide doublers, which are additional layers of composite material, in the vicinity of the fittings in order to increase the bearing surface area and thus reduce the bearing stress. A disadvantage of this technique is that in order for the composite structure to be sufficiently strengthened, the doublers must be relatively thick in comparison to the rest of the structure. This increases the overall size and weight of the structure, so as to significantly reduce the advantages of its use.
Still other techniques to provide composites with fittings have included using adhesives to bond fittings in place. Bonded joints have proven unsatisfactory because many adhesives and matrix materials exhibit a rather low shear strength and in many situations would be exposed to relatively high shear forces. Furthermore, bond joints only work well within narrow temperature ranges. This is due to both the breakdown of the adhesive and because differences in the thermal expansion-contraction characteristics of the adjacent metal and composite stress the bond. Bond joints are also known to break down when subjected to contaminants, such as hydraulic fluid, which are unavoidably present in many mechanical environments. There have also been attempts to bolt or mold fittings to composite structures. Bolting fittings into place requires drilling into the composite, which, as previously discussed, almost always results in its weakening. Composite structures with fittings molded in place are typically subjected to both tension and compression forces which must be sheared into the composite along the composite fitting interface. This interface is limited in size by the ultimate shear strength of the matrix material.
Moreover, bonded fittings, bolted fittings, and molding fittings each require that the actual metal fitting be large-sized in order to ensure that the shear surface or bearing surface is large enough to adequately dissipate the loads throughout the composite structure. This significantly increases the weight of these assemblies. Consequently, the weight savings of composite structure-fitting assemblies becomes trivial when compared to all metal structures, and the advantages of using such assemblies becomes marginal.